Liquid transfer articles such as plates and rolls may be printing plates or rolls or may be metering plates or rolls used in offset printing, flexography or gravure, or plates and rolls used to apply liquid such as ink, lacquer or liquid resin to surfaces to be coated with such liquids. In all cases the liquid or ink to be transferred is held in a plurality of cells engraved in the surface of the plate or roll with the amount of liquid to be transferred depending on the number, size and depth of the cells per unit area of the engraved surface. In practice, the cells are formed close together in a definite pattern of parallel lines of cells. In the case of a cylinder they are formed in a continuous spiral on the cylindrical surface. While the engraved surface can be in the form of a plate, by far the most common and usual form is that of a cylinder having a continuous spiral of cells engraved on its surface. The spiral of cells is engraved so that a plurality of parallel lines of cells are produced all forming the same angle (usually called the screen angle) with a surface line parallel to the longitudinal axis of the cylinder or roll, which angle may be 0.degree.-90.degree., for example, 30.degree. or 60.degree. but more commonly 45.degree..
While a variety of engraved surfaces are used for transferring liquid, ceramic and metal carbide surfaces, because of their extreme hardness and consequent excellent wear resistance, are generally used for liquid transfer. Because of their hardness, however, the only practical method of engraving such surfaces is by means of a laser beam. Although continuous laser beams are used for engraving some surfaces, the pulsed laser beam is much more commonly employed as it lends itself to the production of discreet cells in the engraved surface.
In the conventional method each pulse of the laser beam produces one cell. In producing the cell, however, the pulse also produces a raised portion (i.e. above the original surface of the roll) called "recast" around the periphery of each cell which is generally annular in shape but of irregular width and height. For reasons which will be more fully explained hereinafter the recast makes it impossible by conventional methods to reduce the distance between cells along the spiral engraving line to less than about twice that along a line perpendicular to the longitudinal axis of the cylindrical roll. The recast may be partially or entirely eliminated by polishing of the engraved roll.
Because of the irregularity in the height of the recast, movement of liquid between adjacent cells is possible if no after polishing or insufficient polishing to reduce the recast to flat lands between the cells is undertaken. When it is possible for liquid to pass between adjacent cells this type of structure is termed "an open cell structure". When sufficient polishing is effected to produce flat lands of recast of the same height above the original surface or to the original surface itself, then the cells become closed as the movement of liquid between them is no longer possible.
The general form of the cells in conventional engraving is substantially conical. This creates a problem in that ink or other liquid tends to dry out in the bottom of the cells which, over a period of time, reduces the liquid holding capacity of the engraved roll overall. Also, because the onset and degree of the drying out varies from cell to cell unevenness can result in the transfer of liquid from different parts of the roll. This necessitates periodic cleaning of the engraved rolls which has been found to be time consuming and expensive.
Increasing the depth of the cells by conventional pulsed laser beam techniques with the object of increasing the liquid holding capacity of an engraved roll is self-defeating since the drying out effect places a limit on the depth which can in practice be realized for any given diameter of cell without that effect occurring. Thus if the ratio of diameter/depth is less than 3 times, this drying out problem can occur. For any given diameter of cell beyond a certain depth, the deeper the depth, the more the drying out effect will occur. Also the conical shape of the cells does not lend itself to complete transfer of liquid from the cells when the cells are beyond a certain depth, that depth depending on the diameter of the cells. In practice therefore, increasing the depth of the cells does not really provide an effective increase in the holding capacity or transfer capacity of a roll. If the cells are not thoroughly cleaned, mixtures of different inks and solvents are formed in the cell bottom which can lead to hard deposits being formed. The deeper the cells, the more difficult it becomes to effect thorough cleaning, and the more likely that such deposits will be formed. This offsets any temporary advantage which is achieved by an increased depth of cell in terms of holding capacity or transfer capacity.
Where an open cell structure is employed it is not so important to have all cells equidistant from each other in all directions. Such an open cell structure has advantages when the rolls are themselves printing rolls and are being used to produce substantial areas of block printing of uniform colors. It is disadvantageous when the roll is essentially being used as an ink transfer roll to the screen dot structure on a printing roll. Particularly, when small dots are involved, an open cell structure would enable ink to flow from adjacent cells not actually in contact with the surface of a particular dot into the cells which are in contact with the dot, resulting in an excessive application of ink. This problem is even greater with large size cells in the engraved roll than with small cells. With large cells the amount of ink which can be transferred from cells adjacent but not in contact with the surface of a particular dot would be substantial in the sense that the cells themselves would hold more ink and the number of cells in contact with a particular dot would be relatively small in number. Proportionately therefore, there would be a greater excess of ink under these circumstances than there would be if the cells were very small.
With an open cell structure therefore difficulties would be experienced by a printer in those areas which produced light shades of color since the dots in these regions of the printing roll would be small. If on the other hand a closed cell structure were used, there would be no intrinsic problem since there would be no transfer of ink from adjacent cells into those cells in contact with any particular dot on the printing roll.
Where the printer is required to print both intense colors and very pale shades at the same time the problem becomes acute. If an open cell structure is used with large cells this would enhance the intensity of the deep shades but would create problems in the pale shades. Reducing the size of the cells would reduce the problem in the lighter shaded areas but because of the lower ink holding capacity of a roll with fine or small diameter cells the maximum intensity of color which could be achieved is limited.
As indicated, the use of a closed cell structure would enable larger cells to be used which would have a greater ink transfer capacity to produce the desirable deep shades while having no intrinsic problems in the lighter shaded areas. However, a closed cell structure produced by conventional means would not have the cells spaced from each other equally in all directions and this would itself create problems in the evenness of printing particularly in the deep shaded areas.
Because of the drying out effect mentioned above increasing the depth of individual cells would not in general increase the ink transfer capacity since although this would initially increase to some extent the transfer of ink, it would generally not continue for very long at the initial level once the ink started to dry in the bottom of the conically shaped cell.
It is therefore an object of the present invention to provide a method of engraving using a pulsed laser beam which enables the cells to be formed substantially equidistant from one another in all directions and at the same time have a greater volume without increasing the depth.
It is another object of the present invention to provide a method of producing pulsed laser beams that can impart a pattern of laser engraved cells to a liquid transfer article in which the cells are substantially equidistance apart.
It is another object of the present invention to provide a method for imparting pulsed laser engraved cells in a liquid transfer article in which the cells have an increased volume and are spaced relatively close together.
It is another object of the present invention to produce spaced-apart cells in a liquid transfer article in which each cell is formed from at least two consecutive spaced-apart laser pulses.
It is another object of the present invention to provide a liquid transfer article having a plurality of spaced-apart cells in which each cell is substantially equidistant from each of its adjacent cells.
The above and further objects and advantages of this invention will become apparent upon consideration of the following description thereof.